Typed and Targeted Property Change Events in Java

Targeted Property Change Events

Our current scheme works well for changing automobile engines, and for
objects with simple properties the standard JavaBeans bound property paradigm
gets the job done. But for complex propertiesthose properties the values of
which have their own properties that can changethis paradigm has it
limitations. What if some outside object want to listen for changes in the
engine size as well, for example?

One solution would be for the external object to register a property change
listener directly with the Automobile's engine property value. This
quickly becomes cumbersome, because the value of the engine
property can change. The external class must therefore also register a property
change listener with the Automobile instance, listening for the
"engine" property to change, and then unregister its event listener
from the old engine and register it with the new one. Remembering to do this
(not to mention documenting that this must be done) becomes unmanageable.
Wouldn't it be better if we could do this once in the Automobile
class, and propagate any property changes in the Engine to the
property change listeners of Automobile?

This is a valid approach, but it leaves one detail unresolved: How does a
PropertyChangeListener determine whether a property change was
performed on the Automobile or on its contained
Engine? We could have the Automobile forward the
Engine property change events unchanged, so that
PropertyChangeEvent.getSource() would equal the Engine
instance as appropriate, but if one property change listener is listening to
several Automobiles it prevents them from using
PropertyChangeEvent.getSource() to find which
Automobile's Engine changed properties. Besides, the
contract of PropertyChangeEvent.getSource() (as stated by the API
of java.util.EventObject) is that "All Events are constructed with
a reference to the object, the "source", that is logically deemed to be the
object upon which the Event in question initially occurred upon." While this is
slightly ambiguous in our case as to whether "initially occurred upon" refers to
the object that changed its properties or the object that fired the event, it
seems pragmatically useful for a PropertyChangeListener to expect
PropertyChangeEvent.getSource() to refer to the object with which
the listener registered. This would favor the "object that fired the event as
source" interpretation.

So there needs to be a separate method of PropertyChangeEvent
that indicates the object the property of which changed, regardless of which
object is firing the event. This value would stay the same regardless of how
many times the event was propagated. Indeed it is possible the authors of Java
had something similar in mind for the future, for the API of
java.beans.PropertyChangeEvent.getPropagationId() states:

The "propagationId" field is reserved for future use. In Beans 1.0 the sole
requirement is that if a listener catches a PropertyChangeEvent and then fires
a PropertyChangeEvent of its own, then it should make sure that it propagates
the propagationId field from its incoming event to its outgoing
event.

That sounds a lot like the functionality we want, but with no more
information on semantics, and with no conventions relating to this field in
widespread use, it seems foolhardy to hijack this field for own purposes. If
future versions of Java were to use the propagationId field for some other
purpose, it could cause serious backwards-compatibility issues with our code. It
again seems that the safest route is to add in our own extensions to the
JavaBeans component architecture.

We can start by creating an interface that can be mixed in with any event and
that indicates the target of the eventthe object to which the action was
directed that caused the event, whether or not that object actually fired the
event object in question. This is done in
com.garretwilson.event.TargetedEvent, which could be used with any
type of event, such as a property change event or an action event:

/**An interface for an event that knows its target,
or the object to which the event applies
(which may be different than the object that fired the event).
@author Garret Wilson
*/
public interface TargetedEvent
{
/**Returns the object to which the event applies.
This may be a different than <dfn>source</dfn>,
which is the object that generated this event instance.
@return The target of the event, or <code>null</code>
if the event target is not known.
*/
public Object getTarget();
}

Related Articles

We can now make GenericPropertyChangeEvent<V>, which we
created earlier, compatible with our new targeted event framework. We'll add new
copy constructors that will allow new events to be created with different
sources while still maintaining the same target. We'll also upgrade the old
source-only constructors to automatically set the target to the same value as
the source, for those instances in which there is no need to maintain a separate
target and the class is being used traditionally:

/**A property value change event is a Java Beans
property change event retrofitted to use generics
to cast to proper value type.
This event is also <dfn>targeted</dfn>,
specifying an event target which may or may not
be the same as the source object firing this event.
@param <V> The type of property value.
@author Garret Wilson
*/
public class GenericPropertyChangeEvent<V>
extends PropertyChangeEvent implements TargetedEvent
{
/**The target of the event, or <code>null</code>
if the event target is not known.*/
private final Object target;
/**Returns the object to which the event applies.
This may be a different than <dfn>source</dfn>,
which is the object that generated this event instance.
@return The target of the event.
*/
public Object getTarget() {return target;}
/**Source and property name constructor with old and new values.
The target will be set to be the same as the given source.
@param source The bean that fired the event.
@param propertyName The programmatic name of the property
that was changed.
@param oldValue The old value of the property,
or <code>null</code> if no old value is not available.
@param newValue The new value of the property,
or <code>null</code> if the new value is not available.
*/
public GenericPropertyChangeEvent(final Object source,
final String propertyName, final V oldValue, V newValue)
{
this(source, source, propertyName, oldValue, newValue);
}
/**Source, target, and property name constructor
with old and new values.
@param source The bean that fired the event.
@param target The target of the event.
@param propertyName The programmatic name of the property
that was changed.
@param oldValue The old value of the property,
or <code>null</code> if no old value is not available.
@param newValue The new value of the property,
or <code>null</code> if the new value is not available.
*/
public GenericPropertyChangeEvent(final Object source,
final Object target, final String propertyName,
final V oldValue, V newValue)
{
super(source, propertyName, oldValue, newValue);
this.target= target;
}
/**Property change event copy constructor.
@param propertyChangeEvent A property change event the values
of which will later cast to this class' generic type.
*/
public GenericPropertyChangeEvent(final PropertyChangeEvent
propertyChangeEvent)
{
this(propertyChangeEvent.getSource(), propertyChangeEvent);
}
/**Property change event copy constructor
that specifies a different source.
If the property change event is a {@link TargetedEvent},
the target is copied from that event;
otherwise, the given source will be used as the target.
@param source The object on which the event initially occurred.
@param propertyChangeEvent A property change event the values
of which will later cast to this class' generic type.
*/
@SuppressWarnings("unchecked")
public GenericPropertyChangeEvent(final Object source,
final PropertyChangeEvent propertyChangeEvent)
{
this(source, propertyChangeEvent instanceof TargetedEvent
? ((TargetedEvent)propertyChangeEvent).getTarget() : source,
propertyChangeEvent.getPropertyName(),
(V)propertyChangeEvent.getOldValue(),
(V)propertyChangeEvent.getNewValue());
setPropagationId(propertyChangeEvent.getPropagationId());
}
/**@return The old value of the property,
or <code>null</code> if the old value is not available.*/
@SuppressWarnings("unchecked")
public V getOldValue()
{
return (V)super.getOldValue();
}
/**@return The new value of the property,
or <code>null</code> if the new value is not available.*/
@SuppressWarnings("unchecked")
public V getNewValue()
{
return (V)super.getNewValue();
}
}

The Automobile class can now repeat any property changes of the
contained Engine object by creating a new event object with a new
source, but keeping the same target. This is best done using the
GenericPropertyChangeEvent<V> copy constructor above that
takes a different source as one of its arguments. The new
Automobile class is shown below:

There are three important parts to this property change repeater pattern. The
first is the repeater PropertyChangeListener, which can be
registered with any class. It takes whatever property change that has occurred
and refires the event after creating a new event with an updated source of the
object firing the event. (It is important to note that
Automobile.this is used to represent the Automobile
instance as the source rather than the anonymous inner class.) Secondly, the
repeater is registered with the Engine instance as soon as it is
created. Thirdly, because the engine property can change, a
separate listener is registered with the Automobile instance itself
for notification of when the engine property value changes. When
this happens, the repeater is unregistered from the old Engine and
then registered with the new Engine.

Now an external class can be notified of property changes of the
Engine property of the Automobile by registering with
the Automobile either for all property change events or only for
specific Engine-specific properties. If there is ever a question of
whether a property of Automobile or Engine has
changed, this may be resolved by examining
TargetedEvent.getTarget() if the PropertyChangeEvent
in question implements TargetedEvent.

The concept of a targeted event is especially useful when an object can have
an arbitrary number of contained objects over an arbitrary number of
hierarchical levels. As an example, consider the TreeControl
component in the com.guiseframework.component package of the Guise Internet application
framework. A TreeControl contains a root
TreeNodeModel<V> which itself can contain an arbitrary number
of TreeNodeModel<V> children, each of which in turn can
contain other tree nodes, and so on. A common use case is to listen for the
selection by the user of one of the nodes in a tree.

An application simply cannot practically register and unregister a property
change listeners with each TreeNodeModel<V> as it is added or
removed from the tree. The Guise TreeControl therefore uses the
typed, targeted property change architecture described above to propagate
property changes of each TreeNodeModel<V> up the tree
hierarchy until it is finally repeated from the TreeControl. Any
property change event fired from the control will properly indicate the
TreeControl as its source, and indicate the tree node
with the changed property as its target. An application can
therefore listen directly to the TreeControl to be notified when a
tree node is selected, for example:

The application listens for the TreeNodeModel.SELECTED_PROPERTY
changing by registering one property change listener with the
TreeControl instance. Once the property changes, the listener
verifies that a tree node is indeed the object with the changed selection
status. If the new selected status is true, the application may
decide to perform some special action resulting from the selection change,
perhaps based upon the specific tree node target as reported in
propertyChangeEvent.getTarget().

Summary

This JavaBean extension architecture of typed, targeted property change
events not only is simple and powerful, it also is interoperable with the
existing JavaBeans component architecture. Through the use of generics it
obviates the need for tedious manual casting of values while imposing
consistency and internal type checking within a property change listener. By
introducing the concept of an event target, the framework allows facile
propagation of property change events to a single listening point without losing
the identification of the original object the property of which was changed. A
developer can experiment with the examples presented here using the free
development version of the Guise framework library, available at http://www.guiseframework.com/.